Starch has long been used as an adhesive in pigmented coatings for paper (Starch, Chemistry and Technology; Whistler and Paschall, Editors; Vol. II, Ch. 6; pp. 134-145). In using starch as an adhesive in paper coating, there are certain desirable characteristics, such as the viscosity of the coating formulation, the compatibility of starch with other coating ingredients, IGT pick, adhesive strength, and the like.
Paper coating is used to provide better printing, sharper images, and protective covering for paper. Coatings containing clay are widely used. Addition of colored pigments, starch, latex, or other substances supply special properties to coating compositions. However, clay used in paper coatings must be mixed with water to provide proper viscosity, wetting agents, protective colloids, and an adhesive or binder to bind the clay to the paper surface. Casein is an effective colloid or binder for clay and provides workable viscosity. Unfortunately, casein is relatively expensive.
Modified starches have been used to replace casein. To provide comparable binder characteristics and viscosity, it is necessary to use more starch than casein by weight in clay coatings. The major modification to starch has been a reduction in molecular weight by thinning the starch by acids, enzymes, or oxidation.
In recent years, the use of derivatized starches in paper coating has become more desirable due to the unique properties characterizing these starch products. Unfortunately, the use of starch derivatives adds to the cost of the coating formulation. Hydroxyethyl starch is one of the most widely accepted and used starch derivatives for paper coating colors. The use of hydroxyethyl starch in paper coating colors results in improved flow properties, ink receptivity and printing properties (Starch, Chemistry and Technology; Whistler and Paschall, Editors; Vol. 2, Ch. 17; pp. 427-428).
U.S. Pat. No. 3,884,853 relates to an alkali-stable, modified starch for use in paper coating binders as a replacement for protein (casein and delta protein) over a broad pH range. In this patent a cationic starch derivative having quaternary ammonium groups is reacted with maleic anhydride to yield an amphoteric starch having a net anionic charge. Again, unfortunately the cost of derivatization and particularly double derivatization in the case of U.S. Pat. No. 3,884,853 is prohibitive.
Recently Japanese Unexamined Patent Application No. 75,104/76 describes the use of a cross-linked starch product as a replacement for clay in styrene butadiene latex coating compositions for paper.
U.S. Ser. No. 745,145 now U.S. Pat. No. 4,104,213 relates to latex compositions containing a substantially non-birefringent cold-water swelling granular starch product characterized as having less than 10% solubles and having a swelling power such that each 5 g. when dispersed in water and held for 16 hrs. at 25.degree. C. swells from about 20-ml. to about 35-ml. and to processes for manufacture of latex compositions. In U.S. Ser. No. 745,145 the starch specialty product is incorporated directly into the latex composition without any further treatment.
U.S. Ser. No. 780,614 relates to the use of specially-treated, substantially non-birefringent cold-water swelling granular starch product which is added at the wet-end of paper manufacture or sprayed uniformly onto the wet-web or plys of cellulosic pulp. In U.S. Ser. No. 780,614 the specially-treated starch product is not subjected to further modification prior to its application in paper.
U.S. Pat. No. 3,067,064 discloses a process for producing thin-boiling starch materials having swollen unruptured cells characterized by absence of maltese crosses when illuminated by polarized light when viewed under a microscope. The starchy materials are said to be useful for sizing textiles and papers and for glues and adhesives. However, no details are given pertaining to the use of the starch product and paper coating formulations.
Various methods for coating and coating equipment are available in the paper industry. In application of a coating to paper, viscosity of the coating composition may be altered to fit the method of application. In recent times, blade coating is the most widely used method. In this method, paper traveling around a roll passes through the coating color and then a blade smoothes the surface. Roll coating is another method for coating paper which uses a series of rolls to meter or apply the coating to paper. Air knife coating applies an excess coating to the paper which is removed by pressurized air to provide a smooth coating.